Unfortunately Firefox is one of the very last updated browsers left for XP and it's going away in September after 52.3ESR.
32-bit Firefox has had performance issues and glitches for a long time in XP. You couldn't hibernate or even sleep if even one browser tab was open. Very high CPU use compared to the same version on 32-bit Win 7, and features that work in 32-bit Win7+ like WebGL just don't work in XP.
It's really too bad as XP, at least in POSready 2009 guise, will be supported with security updates and MSE definitions until April 2019. I quit using Win 9x 10 years ago when the last browser for it (Firefox 2.0.0.20) was no longer updated. 9x is a dead platform now without even an antivirus or access to old WindowsUpdates...